[0001] The present invention relates to the purification of gases, and more particularly
to a method of purifying flue gases which contain noxious gases such as SO
2.
[0002] Dry sorbent injection (DSI) has been used with a variety of sorbents to remove SO
x and other gases from flue gas. However, DSI has typically been done in the past at
temperatures much lower than 204.4°C (400° F) because equipment material, such as
baghouse media, cannot withstand higher temperatures. For instance, in
US4555391 is disclosed a process for removing S02 from a flue gas, wherein dry sodium sorbent
is injected in the flue gas at a temperature of about 130°C. Additionally, many sorbent
materials sinter or melt at temperatures near or greater than 204.4°C (400° F), which
makes them less effective at removing gases. The reactions products of many sorbent
materials also adhere to equipment and ducts at higher temperatures, which requires
frequent cleaning of the process equipment. To operate at these lower temperatures,
the combustion gases must often be cooled before the sorbent was injected. This is
an undesirable extra process step.
[0003] Thus, there is a need for a sorbent injection method that is effective at removing
SO
x gases at elevated temperatures.
[0004] In one aspect, a method of removing SO
2 from a flue gas stream including SO
2 is provided. The method includes providing a source of trona and injecting the trona
into the flue gas stream. The temperature of the flue gas is between 315.6°C (600°
F) and 482.2°C (900° F). The trona is maintained in contact with the flue gas for
a time sufficient to react a portion of the trona with a portion of the SO
2 to reduce the concentration of the SO
2 in the flue gas stream.
[0005] In another aspect, a system for the removal of SO
2 from a flue gas stream including SO
2 is provided. The system includes a source of trona and a flue gas stream. The system
also includes an injector for injecting the trona into the flue gas stream. The temperature
of the flue gas is between 315.6°C (600° F) and 482.2°C (900° F). The system also
includes an area for maintaining the trona in contact with the flue gas for a time
sufficient to react a portion of the trona with a portion of the SO
2 to reduce the concentration of the SO
2 in the flue gas stream:
[0006] The foregoing paragraphs have been provided by way of general introduction, and are
not intended to limit the scope of the following claims. The presently preferred embodiments,
together with further advantages, will be best understood by reference to the following
detailed description taken in conjunction with the accompanying drawings.
FIG. 1 is a schematic of one embodiment of a flue gas desulfurization system.
FIG. 2 is a graph showing the % SO2 removal as a function of normalized stochiometric ratio (NSR) for trona and sodium
bicarbonate.
FIG. 3 is a graph showing the % SO2 removal as a function of flue gas temperature (in °F) for one embodiment of a flue
gas desulfurization system.
FIG. 4 shows a perforated plate of an electrostatic precipitator after operation in
one embodiment of a flue gas desulfurization system using trona.
FIG. 5 shows a perforated plate of an electrostatic precipitator after operation in
one embodiment of a flue gas desulfurization system using sodium bicarbonate.
[0007] The invention is described with reference to the drawings in which like elements
are referred to by like numerals. The relationship and functioning of the various
elements of this invention are better understood by the following detailed description.
However, the embodiments of this invention as described below are by way of example
only, and the invention is not limited to the embodiments illustrated in the drawings.
[0008] Dry sorbent injection (DSI) has been used as a low cost alternative to a spray dry
or wet scrubbing system for the removal of SO
2. In the DSI process, the sorbent is stored and injected dry into the flue duct where
it reacts with the acid gas. The present invention provides a method of removing SO
2 from a flue gas stream comprising SO
2, preferably by injecting a sorbent such as trona into a flue gas stream to react
with SO
2. Trona is a mineral that contains about 85-95% sodium sesquicarbonate (Na
2CO
3-NaHCO
3-2H
2O). A vast deposit of mineral trona is found in southwestern Wyoming near Green River.
As used herein, the term "trona" includes other sources of sodium sesquicarbonate.
Embodiments in which the source of sesquicarbonate is mined trona are however preferred.
The term "flue gas" includes the exhaust gas from any sort of combustion process (including
coal, oil, natural gas or glass raw material for example). Flue gas typically includes
SO
2 along with other acid gases such as HCl, SO
3, and NO
x.
[0009] A schematic of the process is shown in FIG. 1. The furnace or combustor 10 is fed
with a fuel source 12, such as coal, and with air 14 to burn the fuel source 12. From
the combustor 10, the combustion gases are conducted to a heat exchanger or air heater
40. The outlet of the heat exchanger or air heater 40 is connected to a particulate
collection device 50. The particulate collection device 50 removes particles made
during the combustion process, such as fly ash, from the flue gas before it is conducted
to the gas stack 60 for venting. The particulate collection device 50 may be an electrostatic
precipitator (ESP). Other types of particulate collection devices, such as a baghouse,
may also be used for solids removal. The baghouse contains filters for separating
particles made during the combustion process from the flue gas. Because of the relatively
small particle size used in the process, the trona may act as a precoat on baghouse
filter media.
[0010] The SO
2 removal system includes a source of trona 30. The trona 30 preferably has a mean
particle size between about 10 micron and about 40 micron, most preferably between
about 24 micron and about 28 micron. The trona is preferably in a dry granular form.
A suitable trona source is T-200® trona, which is a mechanically refined trona ore
product available from Solvay Chemicals, Green River, WY. T-200® trona contains about
97.5% sodium sesquicarbonate and has a mean particle size of about 24-28 micron. The
SO
2 removal system may also include a ball mill pulverizer 32, or other type of mill,
for decreasing and/or otherwise controlling the trona particle size on site.
[0011] The trona is conveyed from the trona source 30 to the injector 20. The trona may
be conveyed pneumatically or by any other suitable method. Trona can be easily aerated
for pneumatic transfer. Apparatus for injecting the trona or sodium sesquicarbonate
is schematically illustrated in FIG. 1. Trona injection apparatus 20 introduces the
trona into flue gas duct section 42, which is disposed at a position upstream of the
baghouse inlet and upstream of the heat exchanger 40, if a heat exchanger or preheater
is present. The trona injection system is preferably designed to maximize contact
of the trona with the SO
x in the flue gas stream. Any type of injection apparatus known in the art may be used
to introduce the trona into the gas duct. For example, injection can be accomplished
directly by a compressed air-driven eductor.
[0012] The process requires no slurry equipment or reactor vessel if the trona is stored
and injected dry into the flue duct 42 where it reacts with the acid gas. However,
the process may also be used with humidification of the flue gas or wet injection
of the trona. Additionally, the particulates can be collected wet through an existing
wet scrubber vessel should the process be used for trim scrubbing of acid mist.
[0013] The temperature of the flue gas varies with the location in the injection system
and may also vary somewhat with time during operation. The temperature of the flue
gas where the trona is injected is between 315.6 °C (600° F) and 482.2°C (900° F).
The trona is maintained in contact with the flue gas for a time sufficient to react
a portion of the trona with a portion of the SO
2 to reduce the concentration of the SO
2 in the flue gas stream. The temperature of the flue gas is preferably greater than
332.2 °C (630° F), and most preferably greater than 371.1°C (700° F). The temperature
of the flue gas is preferably less than 426.7 (800° F), and most preferably less than
398.9°C (750° F). The temperature of the flue gas is most preferably between 371.1°C
(700° F) and 398.9°C (750° F).
[0014] The process may also be varied to control the flue gas temperature. For example,
the flue gas temperature upstream of the trona may be adjusted to obtain the desired
flue gas temperature where the trona is injected. Additionally, ambient air may be
introduced into the flue gas stream and the flue gas temperature monitored where the
trona is injected. Other possible methods of controlling the flue gas temperature
include using heat exchanges and/or air coolers. The process may also vary the trona
injection location or include multiple locations for trona injection.
[0015] For the achievement of desulfurization, trona is preferably injected at a rate with
respect to the flow rate of the SO
2 to provide a normalized stoichiometric ratio (NSR) of sodium to sulfur of between
about 1.0 and 1.5. The NSR is a measure of the amount of reagent injected relative
to the amount theoretically required. The NSR expresses the stoichiometric amount
of sorbent required to react with all of the acid gas. For example, an NSR of 1.0
would mean that enough material was injected to theoretically yield 100 percent removal
of the SO
2 in the inlet flue gas; an NSR of 0.5 would theoretically yield 50 percent SO
2 removal. SO
2 neutralization requires two moles of sodium per one mole of SO
2 present.
[0016] Unlike sodium bicarbonate, trona does not melt at elevated temperatures. Rather,
sodium sesquicarbonate undergoes rapid calcination of contained sodium bicarbonate
to sodium carbonate when heated at or above 135°C (275°F). It is believed that the
"popcorn like" decomposition creates a large and reactive surface by bringing unreacted
sodium carbonate to the particle surface for SO
2 neutralization. The byproduct of the reaction is sodium sulfate and is collected
with fly ash. The chemical reaction of the trona with the SO
2 is represented below:
2 [Na
2CO
3· NaHCO
3 · 2H
2O] → 3Na
2CO
3 + 5H
2O + CO
2
Na
2CO
3 + SO
2 → Na
2SO
3 + CO
2
Na
2SO
3 + 1/2O
2 → Na
2SO
4
The solid reaction products of the trona and the SO
2 (primarily sodium sulfate) and unreacted soda ash may be collected in an electrostatic
precipitator, or other particulate collection device. The total desulfurization is
preferably at least about 70%, more preferably at least about 80%, and most preferably
at least about 90%.
[0017] In one embodiment, the flue gas stream further comprises SO
3. The trona is maintained in contact with the flue gas for a time sufficient to react
a portion of the trona with a portion of the SO
3 to reduce the concentration of the SO
3 in the flue gas stream. SO
3 is typically more reactive with the sorbent than SO
2, so the trona would remove the SO
3 first. The chemical reaction of the trona with the SO
3 is represented below:
2 [Na
2CO
3 • NaHCO
3 • 2H
2O] → 3Na
2CO
3 + 5H
2O + CO
2
Na
2CO
3 + SO
3 → Na
2SO
4 + CO
2
[0018] The trona injection system may also be combined with other SO
x removal systems, such as sodium bicarbonate, lime, limestone, etc. in order to enhance
performance or remove additional hazardous gases such as HCl, NO
x, for example.
EXAMPLES
[0019] A study was done in a commercial glass plant in Verona, CA using a hot side electrostatic
precipitator (ESP) and no baghouse. Natural gas was used as a fuel source, and the
source of sulfur was from the glass raw materials. The SO
2 concentration in the flue gas was 800 ppm. The trona used was T-200® from Solvay
Chemicals. The trona was injected in the duct using a compressed air blower and air
lock feeder. Trona flow rates were measured by calibrating the airlock rpm with the
trona weight loss in the trona storage bin. Trona feed rates varied from 22.7 to 95.7
kg/h (50 to 211 pounds/hr).
EXAMPLE 1
[0020] Trona was injected into flue gas at a temperature of 398.9 °C (750° F) at NSR values
of 1.0, 1.2, and 1.4. FIG. 2 shows the % SO
2 removal as a function of normalized stochiometric ratio (NSR) for trona. From these
tests it can be seen that trona yielded SO
2 removal rates of around 80% at an NSR of 1.2. FIG. 4 shows a perforated plate of
an ESP in the glass plant after operation of the SO
2 removal system for five months using trona. It can be seen that the plate is relatively
free of solids buildup.
EXAMPLE 2
[0021] As a comparative example, sodium bicarbonate was injected under the same conditions
as Example 1 at an NSR of 1.2. The result is shown in FIG. 2. The % SO
2 removal of 72% was significantly lower than that of the trona at the same temperature
and NSR. FIG. 5 shows a perforated plate of an ESP in the glass plant after operation
of the SO
2 removal system using sodium bicarbonate. It can be seen that the plate has significant
solids buildup.
EXAMPLE 3
[0022] Trona was injected into flue gas at a NSR of 1.5 in a temperature range of 398.9
°C (750° F) to 429.4°C (805° F). FIG. 3 shows the % SO
2 removal as a function of flue gas temperature. From these tests it can be seen that
trona yielded SO
2 removal rates of up to 91% and was effective over a wide range of elevated temperatures.
[0023] From the above experiments it can be seen that trona was more effective than sodium
bicarbonate at removing SO
2 from a flue gas stream at elevated temperatures. Thus, the system can use less sorbent
material than a sodium bicarbonate system to achieve the same sulfur reduction. Additionally,
it can be seen that trona had good performance over a wide range of elevated temperatures.
Finally, the SO
2 removal system using trona had much less solids buildup in the perforated plates
of the ESP than a system using sodium bicarbonate.
[0024] The embodiments described above and shown herein are illustrative and not restrictive.
The scope of the invention is indicated by the claims rather than by the foregoing
description and attached drawings. The invention may be embodied in other specific
forms without departing from the invention as set forth in the claims. Accordingly,
these and any other changes which come within the scope of the claims are intended
to be embraced therein.
1. A method of removing SO
2 from a flue gas stream comprising SO
2, comprising:
- providing a source of trona;
- injecting the trona into the flue gas stream, wherein the temperature of the flue
gas is between 315.6°C (600° F) and 482.2 °C (900° F); and
- maintaining the trona in contact with the flue gas for a time sufficient to react
a portion of the trona with a portion of the SO2 to reduce the concentration of the SO2 in the flue gas stream.
2. The method of claim 1 wherein the mean particle size of the trona is less than about
40 micron.
3. The method of claim 1 wherein the mean particle size of the trona is between about
10 micron and about 40 micron.
4. The method of claim 1 wherein the mean particle size of the trona is between about
24 micron and about 28 micron.
5. The method of claim 1 wherein the temperature of the flue gas is greater than 332.2°C
(630° F).
6. The method of claim 1 wherein the temperature of the flue gas is greater than 371.1°C
(700° F).
7. The method of claim 1 wherein the temperature of the flue gas is less than 426.7°C
(800° F).
8. The method of claim 1 wherein the temperature of the flue gas is less than 398.9 °C
(750° F).
9. The method of claim 1 wherein the temperature of the flue gas is between 371.1 °C
(700° F) and 398.9 °C (750° F).
10. The method of claim 1 wherein the trona is injected at a rate with respect to the
flow rate of the SO2 to provide a normalized stoichiometric ratio of sodium to sulfur of between about
1.0 and 1.5.
11. The method of claim 1 wherein the trona is injected as a dry material.
12. The method of claim 1 further comprising milling the trona to a desired mean particle
size at a location proximate the flue gas stream.
13. The method of claim 1 further comprising collecting a reaction product of the trona
and the SO2 in an electrostatic precipitator.
14. The method of claim 1 wherein the flue gas stream further comprises SO3, further comprising maintaining the trona in contact with the flue gas for a time
sufficient to react a portion of the trona with a portion of the S03 to reduce the
concentration of the SO3 in the flue gas stream.
15. The method of claim 1 further comprising adjusting the flue gas temperature upstream
of the trona to obtain the desired flue gas temperature where the trona is injected.
16. The method of claim 15 wherein the adjusting further comprises introducing ambient
air into the flue gas stream and monitoring the flue gas temperature where the trona
is injected.
17. The method of claim 15 wherein the adjusting further comprises controlling the flow
of a material through a heat exchanger in communication with the flue gas.
1. Verfahren zum Entfernen von SO
2 aus einem Abgasstrom, der SO
2 umfasst, umfassend:
- Bereitstellen einer Quelle von Trona;
- Injizieren des Trona in den Abgasstro m, wobei die Tem peratur des Abgases zwischen
315,6 C (600 F) und 482,2°C (900°F) beträgt; und
- Halten des Trona in Kontakt mit dem Abgas über eine Zeit, die ausreicht, um einen
Teil des Trona mit einem Teil des SO2 umzusetzen, um die Konzentration des SO2 in dem Abgasstrom zu verringern.
2. Verfahren gemäß Anspruch 1, wobei die mittlere Partikelgröße des Trona weniger als
etwa 40 Mikrometer beträgt.
3. Verfahren gemäß Anspruch 1, wobei die mittlere Partikelgröße des Trona zwischen etwa
10 Mikrometer und etwa 40 Mikrometer beträgt.
4. Verfahren gemäß Anspruch 1, wobei die mittlere Partikelgröße des Trona zwischen etwa
24 Mikrometer und etwa 28 Mikrometer beträgt.
5. Verfahren gemäß Anspruch 1, wobei die Temperatur des Abgases mehr als 332,2 °C (630
°F) beträgt.
6. Verfahren gemäß Anspruch 1, wobei die Temperatur des Abgases mehr als 371,1 °C (700
°F) beträgt.
7. Verfahren gemäß Anspruch 1, wobei die Temperatur des Abgases weniger als 426,7 °C
(800 °F) beträgt.
8. Verfahren gemäß Anspruch 1, wobei die Temperatur des Abgases weniger als 398,9 °C
(750 °F) beträgt.
9. Verfahren gemäß Anspruch 1, wobei die Temperatur des Abgases zwischen 371,1 C (700
F) und 398,9°C (750°F) beträgt.
10. Verfahren gemäß Anspruch 1, wobei das Trona mit einer Rate bezogen auf die Flussrate
des SO2 injiziert wird, die ein normalisiertes stöchiometrisches Verhältnis von N atrium
zu Schwefel zwischen etwa 1,0 und 1,5 bereitstellt.
11. Verfahren gemäß Anspruch 1, wobei das T rona als T rockenmaterial injiziert wird.
12. Verfahren gemäß Anspruch 1, ferner umfassend Mahlen des Trona auf eine gewünschte
mittlere Partikelgröße an einem Ort nahe dem Abgasstrom.
13. Verfahren gemäß Anspruch 1, ferner um fassend Sammeln eines Reaktionsprodukts des
Trona und des SO2 in ein em elektrostatischen Präzipitator.
14. Verfahren gemäß Anspruch 1, wobei der A bgasstrom ferner SO3 umfasst, ferner umfassend Halten des Trona in Kontakt mit dem Abgas über eine Zeit,
die ausreicht, um einen Teil des Trona mit einem Teil des SO3 umzusetzen, um die Konzentration des SO3 in dem Abgasstrom zu verringern.
15. Verfahren gemäß Anspruch 1, ferner um fassend Einstellen der Abgastemperatur dem Trona
vorgeschaltet, um die gewünschte Abgastemperatur zu erhalten, wo das Trona injiziert
wird.
16. Verfahren gemäß Anspruch 15, wobei das Einstellen ferner Einführen von Umgebungsluft
in den Abgasstrom und Überwachen der Abgastemperatur, wo das Trona injiziert wird,
umfasst.
17. Verfahren gemäß Anspruch 15, wobei das Einstellen ferner Steuern des Flusses eines
Materials durch eine n Wärmeaustauscher in Verbindung mit dem Abgas umfasst.
1. Procédé d'abattement de SO
2 d'un flux de gaz de combustion comprenant du SO
2, comprenant:
- l'obtention d'une source de trona;
- l'injection du trona dans le flux de gaz de combustion, la température du gaz de
combustion se situant entre 315,6°C (600°F) et 482,2°C (900°F) ; et
- le maintien du trona en contact avec le gaz de combustion pendant un temps suffisant
pour faire réagir une partie du trona avec une partie du SO2 afin de réduire la concentration du SO2 dans le flux de gaz de combustion.
2. Procédé selon la revendication 1 dans lequel la taille moyenne de particules du trona
est inférieure à environ 40 µm.
3. Procédé selon la revendication 1 dans lequel la taille moyenne de particules du trona
se situe entre environ 10 µm et environ 40 µm.
4. Procédé selon la revendication 1 dans lequel la taille moyenne de particules du trona
se situe entre environ 24 µm et environ 28 µm.
5. Procédé selon la revendication 1 dans lequel la température du gaz de combustion est
supérieure à 332,2°C (630°F).
6. Procédé selon la revendication 1 dans lequel la température du gaz de combustion est
supérieure à 371,1°C (700°F).
7. Procédé selon la revendication 1 dans lequel la température du gaz de combustion est
inférieure à 426,7°C (800°F).
8. Procédé selon la revendication 1 dans lequel la température du gaz de combustion est
inférieure à 398,9°C (750°F).
9. Procédé selon la revendication 1 dans lequel la température du gaz de combustion se
situe entre 371,1°C (700°F) et 398,9°C (750°F).
10. Procédé selon la revendication 1 dans lequel le trona est injecté à un débit par rapport
au débit du SO2 permettant d'obtenir un rapport stoechiométrique normalisé du sodium au soufre entre
environ 1,0 et 1,5.
11. Procédé selon la revendication 1 dans lequel le trona est injecté sous la forme d'une
matière sèche.
12. Procédé selon la revendication 1 comprenant en outre le broyage du trona jusqu'à une
taille moyenne de particules souhaitée à un endroit proche du flux de gaz de combustion.
13. Procédé selon la revendication 1 comprenant en outre la collecte d'un produit de réaction
du trona et du SO2 dans un filtre électrostatique.
14. Procédé selon la revendication 1 dans lequel le flux de gaz de combustion comprend
également du SO3, comprenant en outre le maintien du trona en contact avec le gaz de combustion pendant
un temps suffisant pour faire réagir une partie du trona avec une partie du SO3 afin de réduire la concentration du SO3 dans le flux de gaz de combustion.
15. Procédé selon la revendication 1 comprenant en outre l'ajustement de la température
du gaz de combustion en amont du trona pour obtenir la température du gaz de combustion
souhaitée là où le trona est injecté.
16. Procédé selon la revendication 15 dans lequel l'ajustement comprend en outre l'introduction
d'air ambiant dans le flux de gaz de combustion et la surveillance de la température
du gaz de combustion là où le trona est injecté.
17. Procédé selon la revendication 15 dans lequel l'ajustement comprend en outre le contrôle
du débit d'une matière à travers un échangeur de chaleur en communication avec le
gaz de combustion.